In order to meet the demand for reduction in size and weight of portable electronic equipment, increase of capacity of the battery used therefor is deemed the most important subject for improving battery performance. Development and improvement of a variety of batteries have been proceeding along this line. Of the batteries developed to date, lithium ion secondary batteries are expected to achieve the highest capacity and have been and will be given intense improvement.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The ion conducting layer used in a lithium ion secondary battery now available for practical use is a separator made of a porous film of polypropylene, etc. filled with an electrolytic solution.
In the lithium ion secondary batteries now available for practical use, electrical connections among the positive electrode, the ion conducting layer, and the negative electrode are maintained by pressure application by using a firm battery case made of stainless steel, etc. However, such a case increases the weight of a lithium ion secondary battery, making it difficult to realize size and weight reduction. Moreover, the rigidness of the case narrows the freedom of shape design.
In order to realize size and weight reduction and freedom of shape design of a lithium ion secondary battery, it is necessary to join an ion conducting layer to a positive and a negative electrode and to maintain the joined state without applying pressure from the outside.
In this connection, U.S. Pat. No. 5,437,692 discloses a structure in which a lithium ion-conducting polymer is used as an ion conducting layer, and a positive electrode and a negative electrode are joined to the ion-conducting layer with an adhesive layer containing a lithium compound. WO95/15589 discloses a structure having a plastic ion-conducting layer to which a positive and a negative electrode are joined.
According to the method disclosed in U.S. Pat. No. 5,437,692 supra, however, the joint strength attained is not enough, the battery cannot be made sufficiently thin, and the ion conduction resistance between the positive and the negative electrodes through the ion-conducting layer is high so that the battery characteristics such as charge and discharge characteristics are insufficient for practical use. According to WO95/15589 supra, the ion-conducting layer, being plastic, cannot secure sufficient joint strength, and the thickness of the battery cannot be reduced sufficiently.
The present invention has been made in order to solve these problems. It provides a process for producing a lithium ion secondary battery in which a positive and a negative electrode are brought into intimate contact with an ion-conducting layer (a separator) with an adhesive resin to secure sufficient joint strength among the electrodes and the separator while suppressing ion conduction resistance among them on the same level as in a conventional battery put in a case.